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Trying to learn steam heat terminology, can anyone explain what the difference is between a cycle,cycling and short cycling ? Thanks


  • FredFred Posts: 7,916Member
    A cycle is that period of time a boiler is either heating or not heating. When running, that's a heat cycle, when idle, that's an off cycle.
    Cycling is a series of cycles and
    Short cycling is when a boiler shuts down because it has reached a Pressure threshold, rather than the thermostat being satisfied that the room temp is at the tstat setting. Usually happens because a boiler is over-sized and produces more steam than the radiators can condense or because the main venting is poor and the boiler spends a lot of upfront time trying to push the air out of the system through small radiator vents, rather than large main vents. That extended time contributes to pressure build up. Typically the boiler runs three to five minutes, burner is shut down by the Pressuretrol, pressure drops in three or four minutes and burner starts up again. This can occur multiple times during a heating cycle.
  • Jamie HallJamie Hall Posts: 10,661Member
    All true enough. But it is critically important to distinguish between cycling on pressure which occurs near the beginning of a call for heat, or continuously during a call, and cycling on pressure which occurs after a fairly long call -- often as much as thirty minutes to an hour, such as might happen recovering from a setback.

    The causes and cures are completely different.

    The former is caused by inadequate venting, and can be sharply reduced, if not usually eliminated entirely, by adding main venting.

    The latter comes when the radiators are all full and condensing all the steam they can handle, and the boiler is producing more than that -- and thus has to turn off briefly. The cure there is much more difficult. Sometimes one can downfire the boiler enough to stop it (although there are definite downsides to that) but if the boiler is really oversize, there's no easy cure.

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
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